President Trump’s decision to negotiate a break for China on tariffs is galling for Japan, which is reeling from auto sector levies that the White House has shown no sign of willingness to lift. Japan, a top U.S. ally in Asia, was eager to advance trade negotiations with Washington, even as Mr. Trump imposed tariffs on automobiles and threatened an across-the-board 24 percent tariff on Japanese goods. While Beijing and others assembled plans for retaliatory tariffs, Japan rushed to Washington for trade negotiations, armed instead with commitments to buy more American goods and boost investments in the United States to $1 trillion. In Tokyo, the sting of these unsuccessful attempts at diplomacy is becoming palpable. On Tuesday — one day after the Trump administration agreed to temporarily nix most of its tariffs on China — two of Japan’s top automakers issued dire profit forecasts, weighed down by billions of dollars in losses linked to U.S. car tariffs.